Thermal insulation



L. C. MATSCH Nov. 21, 1961 Reflected Radiation -Incom|ng Radlahon Nov.21, 1961 1 c. MATscH 3,009,601

THERMAL INSULATION Filed July 2, 1959 4 Sheets-Sheet 2 INVENTOR LADISLASC.MATSCH A TTORN Y Nov. 21, 1961 L. C. MATSCH THERMAL INSULATION FiledJuly 2, 1959' APPARENT THERMAL coNoUcTlvlTY, K, BT%HR XFT XF) X ,0-1

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LADISLAS C. MATSCH BY Nov. 21, 1961 1 c. MATscH THERMAL INSULATION 4Sheetsheet 4 Filed July 2, 1959 NFL r E .m4

0 OOI INVENTOR. LADISLAS C. MATSCH MKZ/53% United States Patent Oihce3,009,601 Patented Nov. 2l, 1961 3,009,601 THERMAL INSULATION LadislasC. Matsch, Kenmore, N .Y., assignor to Union Carbide Corporation, acorporation of New York Filed `Iuly 2, 1959, Ser. No. 824,690 2 Claims.(Cl. 220-9) This invention relates to an improved insulation having ahighresstance to all modes of heat transfer, and particularly concerns alow temperature, heat insulating material adapted to improve a vacuuminsulating system.

In the conservationand conveying of low temperature commercialproducts-for example, perishable commodities which must be held at lowtemperatures for substantial periods of time, and valuable volatilematerials, such as liquefied gases having boiling points at atmosphericpressure below 233 K., for example, liquid oxygen or nitrogen-a majorproblem encountered is the control of heat leak to the material, whichin the case of liquefied gases results in loss due to evaporation. Inthe conventional double walled liquid-oxygen container, the spacebetween the walls is suitably insulated to limit this evaporation loss.However, up to now it has not been possible to provide an insulationsystem, particularly for small, portable containers having small volumesin comparison with the surface areas, which will limit the evaporationloss to satisfactorily lo-w values.

The basic sys-tems for insulating the conventional double walledcontainer for the conveyance and storage of low boiling liquefied gasesare: for small containers, the Dewar type high vacuum-polished metalsurface system, and for large containers, the powder-in-vacuuminsulation system, whichuses an insulating powder in the vacuum spacebetween the `w-alls. This system is described in detail in U.S. Patent2,395,459. Although powder-invacuum heat insulation is highly effectivein reducing thermal heat loss in many systems, it is not as effective`as straight vacuumpolished metal surface for containers up to two feetin diameter. While these systems have greatly affected the commercialconsiderations las applied to storage and conveyance of low temperatureproducts, there, nevertheless, exists a gre-at commercial need for moreefiicient insulating materials capable off meeting more rigid andexacting requirements, and which will provide even lower thermalconductivities than those afforded lby either of the abovedescribedinsulations.

To give some insight into the problems that are presented in effectingyfurther reductions in heat leak for small portable containers, assumefor example that it is desired 4to insulate a double walled cylindricalcontainer for low boiling 'liquefied gases such as oxygen, so that theevaporation loss due to heat leak will be less than 1% of the containedmaterial per day. Assume further that the container will havehemispherical ends, an inner vessel diameter of 8 inches and an innervessel total length of 48 inches. Using one of the best insulatingmaterials of the prior art, for example, powder-in-vacuum insulation, inaccordance with U.S. Patent 2,396,459, the vacuum being on the order of0.1 micron of mercury absolute, a thermal conductivity off 9.2 X 104B.t.u./ (hr.) (ft.) F.) may be achieved. In order to more fullyappreciate the significance of such a Ithermal conductivity, theinsulating effects of the following insulation thicknesses are setforth. An insulation thickness of 1.66 inches of a powder-in-vacuuminsulation will permit Ian evaporation loss of 7.1% per day. Such laninsulation thickness results in an insulation cross sectional area equal-to the useful cross sectional area of the inner storage vessel. Inother words, beyond the thickness of v1.66 inches, the bulk of Itheinsulation which must be stored and/or transported becomes greater thanthe bulk of Ithe contained stored material.

Increasing the thickness of such an insulation to 4 inches reduces theloss rate to 3.6% per day, while simul-` taneously increasing theinsulation vollume lto about 3 times that of the storage capacity of theinner vessel. lt has been found that it is'entirely impractical toconsider insulating the vessel with a material having a conductivity ashigh as 9.2 104 B.t.u./ (hr.) (ft.) lF.) since cal# culations show thetheoretical required insulation thick-l ness to be 1-05 inches.

Considering 'a straight vacuum insulating. system in which the walls o-fthe inner vessel and outer casing form-v ing the insulation space arepolished in order to reect radiant heat energy, there is a problem ofmaintaining a sufciently low vacuum to eliminate heat conduction byresidual gas. For this purpose ythe absolute pressure within theinsulating space must be maintained at a value 10 to 100 times lowerthan when a powder-in-vacuum insulating sys-tem is used. The vacuumshould -be less than 0.0i micron of mercury absolute pressure andpreferably should be on the order of 0.001 micron mercury. This may vbeobtainable in special laboratory equipment, but is an impracticalspecification 'for fabricated metal vessels intended for industrialservice. Assuming that low vacuum conditions could be maintained so thatheat trans# mission by conduction through the residual `gas would benegligible, there still remains the problem of `achieving the necessaryreflectivity for the vessel walls. To obtain a maximum storage loss rateof 1% per day, surface reilectivities of at least 99.6% must 'beobtained. Reflectivities of this order are only obtainable, if at all,under strictly controlled laboratory conditions, which may not lbeduplicated or maintained either during fabrication of the container, orafter the container is in service.

A lower quality reflective surface may be tolerated by interposingseveral concentric reflective shields within the insulation space asdescribed in U.S. Patent 2,643,022. However, one of the llimitingdiflcullties involved in such an arrangement is in assembling andsupporting many reflective shields Within a reasonable insulationthickness so that each shield is properly spaced from adjacent shieldsat -all points. Proper spacing is an 'absolute necessity, for if twoadjacent shields are permitted to contact in even a minute area, theinsulating effect of one shield will be essentially eliminated.Moreover, the number of shields required `depends on their surfacereflectivity. If a very highly polished surface is provided for bothvessel walls and the shields, the polished surface having a rciiectivityof then at least 10 such shields must be used in'order to achieve a 1%per day storage loss rate in the described vessel. At the same time, tomaintain a reasonable thickness of insulation, the shields must bespaced as close to` gether as possible. Allowing for inaccuraciesinforming and assembling the shields, the spacing of at least 1A inchwould appear to be reasonable. Ten shields between the container wallswould provide 1l spaces, and taking the thickness of the shields intoconsideration, would account for an overall thickness of at least 3inches. Under these circumstances the fabrication of vessels having -astorage loss rate of less than 1% per day would be costly andtimeconsuming.

In view of these obstacles, it has heretofore been impossible toapproach, much less achieve, heat leaks of such small quantities forsystems involving extended periods of storage of low boiling liquefiedgases in portable storage containers.

It is, therefore, an important object of the present invention toprovide a greatly improved insulation system for reducing heattransmsision by all modes of heat transfer to values well below that ofany previously known insulating system.

Another object of the present invention is to provide a novel insulatingmaterial in an insulation system where radiation would otherwise be animportant mode of heat transfer.

Another object of the invention is to provide in a low heat conductivematerial wherein radiation is the predominant remaining mode of heattransfer, a multiplicity of parallel radiant heat barriers interposed insaid low conductive material for substantially reducing the passage ofradi-ant heat therethrough.

Yet another object of the invention is to provide in a low heatconductive insulation, a series of spaced, heat reflecting barriers soconstructed and arranged as to impede the passage of radiant heatthrough said insulation without affecting the thermal conductivitythereof.

Another object of the present invention is to provide in a restrictedgas-evacuated insulating space, a plurality of radiation barriers, saidbarriers being disposed in spaced relation to each other, and maintainedin such spaced position by a low heat conductive spacing material.

Still another object of the present invention is to provide in avacuum-solid insulating space for small portable containers, amultiplicity of radiation barriers comprising spaced and parallel foilsof heat refiective material for reducing the transfer of heat byradi-ation, and a spacing material between said radiation barriers,comprising a low-conductive, heat insulating material for reducing thetransfer of heat by conduction between said barriers.

A further object of the invention is lto provide a mul-tilayer compositeinsulation system in which gas molecules can move transversely throughthe layers, so as to facilitate easier evacuation of such system.

`A still further object of the invention is to provide a vacuum,multi-layer composite insulation system which is superior to heretoforeproposed vacuum insulating systems in impeding heat transfer withoutrequiring the extremely high vacuums associated with straight vacuumsystems.

A further object of the present invention is to provide an improvedmethod of fabricating and applying a heat insulation for cylindricalcontainers wherein the heat insulation comprises a low-conductive, heatinsulating material for reducing the transfer of heat by conduction, andincorporates therein a multiplicity of radiant heat barriers forreducing the transfer of heat by radiation.

A further object of the present invention is to provide in an enclosedvolume defining a gas evacuated insulating space, a novel insulatingstructure adapted to fill the insulating space and effect contact withthe wall surfaces defining the insulating space, said insulating spacebeing characterized by the absence of gross voids, and having a low rateof heat transfer by conduction and radiation.

Other objects, features and advantages of the present invention will beapparent from the following detailed description.

In the drawings:

FIG. l is a front elevational view, partly in section, of adouble-walled liquid gas container embodying the principles of theinvention;

FIG. 2 is an isometric view of the composite insulating material of theinvention shown in a flattened position with parts broken away to exposeunderlying layers;

FIG. 3 is a greatly enlarged detail section view showing the irregularpath of heat transfer through the composite insulating material of theinvention;

FIG. 4 is a sectional view taken along line 4 4 of FIG. l, illustratingthe spiral wrapping of insulating material of the invention;

FIG. 5 is a section view similar to FIG. 4, but showing a concentriclayered modification thereof;

FIG. 6 is a fragmentary elevational view, in section, of a modifieddouble-walled liquid gas container embodying the principles of theinvention;

FIG. 7 is an isometric view similar to FIG. 2, but modified to show acomposite insulating material with perforated foil;

FIG. 8 is a graph showing the effect of web density on the performanceof the present insulating material; and

FIG. 9 is a graph for selecting an optimum insulating material of theinvention for a given system.

In the past, radiation shields used in vacuum spaces have beenconstructed for the most part to be suppor-tingly suspended in spacedrelation to each other. Numerous small diameter supports were employedin the vacuum space to support the insulated vessel and to maintainproper shield spacing. A minimum number of Ithese supports were employedto restrict the passage of heat leak by conduction. The remaining spacewas left unfilled to avoid creating additional pathways for thermalconduction. Furthermore, it was believed that the reflectivecharacteristics of the shields would be seriously impaired by contactwith an insulating filler.

It has been discovered that the insulating qualities of an evacuatedinsulating space may be substantially enhanced to a degree never beforeattained with a novel insulating structure, which may occupy part of orthe entire insulating space. Yet the insulating structure does notrequire numerous brace bars or other supports, does not provide grossvoids Within the insulating structure, and can also be employed as anovel means for elastically supporting the -insulated inner containerMore specifically, it has been discovered that the transmission of heatacross a solid-in-vacuum type insulation may be substantially reduced-to a degree greater than has heretofore been possible by the use of alow heat conductive material which incorporates therein a multiplicityof radiation impervious shields to substantially eliminate heat leak byradiation.

Furthermore, it has been discovered that the placing of refiectiiveshields in direct contact with an insulating material does notsubstantially impair the radiation barrier qualities of the shields.

The term vacuum as used hereinafter is intended to apply tosub-atmosphereic absolute pressure conditions not substantially greaterthan 10 microns of mercury, and preferably below 5 microns of mercury.For superior quality results, the pressure should preferably be below lmicron of mercury.

According to the invention, a vacuum insulated space is provided with alow heat conductive material having incorporated therein a multiplicityof radiation barriers disposed substantially transversely to thedirection of heat ow in spaced relation to each other. Theradiationbarriers or shields of the invention may comprise one or moresheets of heat absorbing material, or preferably thin sheets or layersof a material possessing high reflecting characteristics when exposed toinfra-red radiation, such as aluminum or tin foil. The low conductivematerial also acts as a supporting and spacing material for retainingthe radiation barrier sheets in uniformly spaced relation to each otherindependently of the thickness and stiffness of the barriers. In thismanner it is possible for a large number of thin foils to be supportablymounted and maintained in position in an insulation space of limitedthickness. A clearance of a few thousandths of an inch between foils isenough to effectively interrupt and reflect the radiant heat. In thisway it is possible to provide a large number of shields in a verylimited space, ranging up to several hundred shields per inch ofcomposite insulation thickness.

Shown in FIG. l is a double walled heat insulating container havingparallel inner vessel and outer casing walls 10a and 10b and anevacuated insulating space 11 therebetween. Disposed within theinsulation space 11 is a composite insulation material 12 embodying theprinciples of the invention, and comprising essentially a low heatconductive material 13 having incorporated therein multiple reflectiveshields or radiation barriers 14 in contiguous relation for diminishingthe transfer of heat by radiation across the insulating space l11. Theinsulation appears as a series of spaced refiectors 14 disposedsubstantially transversely to direction of heat ow and supportablycarried by the low-conductive insulating material. The insulatingmaterial uniformly contacts and supports the surface of each radiationshield in superposed relation and, in addition to its primary purpose ofserving as an insulating material, constitutes a carrier and spacingmaterial for maintaining a separation space between adjacent shields. Noother supports are required to maintain the insulation in operativeassembled relation.

The radiation shield material 14 to be used in the insulation material12 of the invention may comprise either a metal or a metal coatedmaterial, such as aluminum coated plastic film, or other radiationreiiective material. Radiation refiective materials comprising thinmetallic foils are admirably suited in the practice of the presentinvention. The foils should have suiiicient thickness to resist tearingor other damage during installation. For high-quality'insulations, thefoil should be as thin as practical consistent with strengthrequirements. Thinness is benecial because it facilitates folding andforming the insulation to fit the contour of the insulation space. Italso minimizes the weight of the container. In cryogenic vessels, lowdensity is additionally important because it reduces the time and thequantity of expensive refrigeration needed to cool down the inner vesseland establish a stable ktemperature gradient through the insulation.Foil thicknesses between 0.2 mm. and 0.002 mm. are suitable, and whenaluminum foil is employed, thickness betwen 0.02 mm. and 0.005 mm. arepreferred.

A preferred reflective shield is 1A mil (0.00025 in. or 0.0062 mm.thick) plain, annealed aluminum foil without lacquer or other coating.Also, any film of oil resulting from the rolling operation should beremoved as by washing. Other radiation reflective materials which aresusceptible of use in the practice of the invention are tin, silver,gold, copper, cadmium or other metals. The ernissivity of the reflectiveshield material should -be between about 0.005 and 0.2, and preferablybetween 0.015 and 0.06. Emissivities of 0.015 to 0.06 (98.5% to 94.2%reiiectivity) are obtainable with aluminum and are preferred in thepractice of this invention, while with more expensive materials such aspolished silver, copper or gold, emissivities as low as .005 may beobtained. The above ranges represent an optimum balance between the highperformance and high cost of low emissivity materials.

In a preferred embodiment, the reflective shields are perforated so asto permit gas in the insulation space to move radially through theinsulation layers, rather than only parallel to the foil layers. Thispermits the gas molecules to migrate more freely towards the evacuationconnection or towards a gas trapping means such as an adsorbent orgetter.

The base or separating material of the invention is a low heatconductive material such as fiber insulation which is provided in anuncompacted, elastically cornpressible, resilient and fluffy state,preferably in the form of sheets. The present low conductive material ispreferably sufficiently compressible `so that the installed density ofsuch mate-rial as an element of the composite insulation is at leasttwice that of the uninstalled material. The physical properties of thismaterial, known as webs to those skilled in the art, must be closelycontrolled to obtain the highly eficient composite insulatingr materialo-f the present invention. It has been found that compressible sheets ofvery fine, low conductive fibers which are matted but unbonded togetherare satisfactory. Resin bonding is frequently employed in themanufacture of fibrous materials but such bonding cannot be tolerated inthe insulation of the present invention because of the resultingexcessive solid conductive path.

Suitable fibers include clean glass filaments having diameters between0.2 and 5 microns such as those produced by the so-called iiameattenuation process. A fiber diameter range of 0.5 to 3.8 microns ispreferred in the practice of this invention. The above ranges representpreferred balances between increasing fragileness and cost of relativelysmall diameter fibers, and increased conductance and gas pressuresensitivity of relatively large diameter fibers, as will be discussedlater in detail. Furthermore, the low conductive separating material ofthis invention preferably comprises fibers which are substantiallyrandomly disposed within the plane in the installed condition, and theindividual 'fibers are also preferably oriented in a directionsubstantially perpendicular to the flow of heat. It will be understoodthat as a practical matter, the fibers will not be individually confinedto a single plane, but rather, in a finite thickness of fibrousmaterial, the fibers will be generally disposed in thin parallel stratawith, of course, some indiscriminate cross weaving of bers betweenadjacent strata. Compressible fibers having diameters in the range of0.75 to 1.5 microns such as those commercially designated as 108 or AAliber, and fibers designated as 1,12 or B fiber having diameters in therange of 2.5 to 3.8 microns are normally prepared as webs, and aresuitable for practicing this invention.

It is to be understood that the compressible, low conductive materialwhich constitutes a preferred element of the present invention does notinclude paper type materials which are relatively smooth,non-compressible, and permanently compacted when provided in the sheetform. |In many systems, the present compressible materials are superiorto paper materials because in several respects, one being that theyminimize the number and size of gross voids in the composite insulationwhen assembled in the compressed state. This means that the pressuresensitivity of the insulation is minimized; that is, the thermalconductivity does not increase at a rapid rate as the pressure in thevacuum space increases.

The reflective shield separating layer must be low conductive in thesense that it presents a high resistance to the flow of heat through thesolid material of which it is composed. While we do not wish to be boundby any particular theory, it is believed the principal reasons for thefar superior insulating effects achieved by the previously describedfiber orientation are the relatively few fibers traversing the thicknessof the insulating layer and the very large number of point contactsestablished between crossing fibers. These point contacts represent thepoints of bearing between adjacent fibers in the direction of heat flow,and as such, constitute an extremely high resistance to the iiow of heatby conduction. In a given thickness of low conductive material, it isclear that more point contact resistances will be present in finecompressible fibers than in coarse fibers. Alternatively, for a givennumber of point contact resistances, fine fibers will permit a thinnerseparating layer than will coarse fibers. This is one important reasonwhy extremely fine compressible fibers are preferred in this invention.

Another reason for using extremely fine compressible yfibers is toreduce gaseous conduction through the insulation and to obtain aninsulation which is relatively insensitive to moderate changes inresidual gas pressure. The larger the particle size (eg. fiber diameter)of the low conductive material, the larger will be the voids between theparticles and the greater will be the heat transfer by solidconductance. Heat is transferred across the voids by molecules of theresidual gas in the insulation space. However, the path of greatestresistance to heat flow is through the individual particles and acrossthe point contacts between the particles. Gas conduction across thevoids may, therefore, be viewed as a short circuit around the principalresistance. The rate of heat transfer by gaseous conduction is dependentupon the number of molecules present and upon the mean-free-path ofmolecular motion. Reducing the absolute pressure reduces the number ofmolecules present to transfer heat, and for this reason, a good vacuumis important. However, reducing the absolute pressure will increase themean-free-path of the molecules and tend to increase gaseous conduction.lf the voids are large so that their average dimension is comparable toor exceed the meanfree-molecular path, then the adverse effect ofincreasing the mean-free-path essentially cancels out the beneficialeffect of fewer molecules. For this reason, reducing the absolutepressure will not reduce gaseous conduction until the mean-free-path haslengthened to the point that molecular motion is restricted by thedimensions of the void spaces. This is why extremely low absolutepressures (e.g., l*6 mm. Hg.) are required in straight vacuum systems orin coarse particle fillers where the dimensions across the void spacesare relatively long. ln such systems, a slight increase in absolutepressure not only increases the number of molecules present but alsoreduces their mean-ree-path so that the voids no longer restrictmolecular motion. The gas then attains its maximum heat carryingcapacity, and the full effect of the short circuit by gaseous conductiondevelops rapidly. In commercial vessels constructed of metal and subjectto rough treatment, is is usually impractical to maintain extremely lowabsolute pressures such as l06 mm. Hg. in the insulation at all times. Avery fine particle compressible material between the shields relaxes thevacuum requirement for the insulation and results in a dependablehighquality insulation system. Accordingly, for the aforementionedreasons it has been found that fiber diameters of between about 0.5 and3.8 microns provide far superior quality insulation than fibers withlarger diameters, and this constitutes the preferred range of thelow-conductive material of the present invention.

The sequence of modes of heat transfer which might occur in a typicalmulti-layer insulation of aluminum foils which are proximately spacedfrom each other by layers of glass liber having a iber orientationsubstantially parallel to the aluminum foils and transverse to thedirection of heat flow, might be as follows:

Referring to FIG. 4, radiant heat striking the first sheet of aluminumfoil will for the most part be reflected, and the remaining partabsorbed. Part of this absorbed radiation will tend to travel toward thenext barrier by reradiation, where again it will be mostly reflected,part will travel by solid conduction, and a minor part by conductionthrough the residual gas. According to the solid conduction method ofheat transfer, the heat leak proceeds along the liber webs in what mightbe considered an irregular path, crossing relatively small areas ofpoint contact between crossing fibers until it reaches the second sheetof aluminum foil, where the heat reflecting iand absorbing processdescribed above is repeated. Because of the particular orientation ofthe individual fibers` in the Webs, the path of solid conduction fromthe first sheet of aluminum foil to the second is greatly lengthened,and encompasses an indefinitely large number of point contactresistances between contacting fibers. By analogy it will be seen that amulti-layer insulation having a series of heat reiiecting sheets and acompressible liber oriented web layer of low conductive insulatingmaterial therebetween may be particularly efcient in preventing ordiminishing heat losses by radiation as well -as by conduction.

In the practice of the present invention, the radiation shield spacingmay be between about per inch using relatively thick webs forseparation, and about 50 shields per inch using very thin webs havingonly a few iibers per unit area of the low conductive layer. A preferredrange is between l0 and 30` shields per inch. These ranges representpreferred balances between the conductive and radiative modes of heattransfer as will now be explained in detail. With a given compressibleweb, the thickness of the layers may be varied considerably by applyingmore or less compression on the layer material during installation.However, it has been unexpectedly discovered that the fine ber materialsof this invention are extremely sensitive to compression, and that arather narrow optimum range exists for the number of radiation shieldsinstalled per unit thickness of composite insulation .as

previously defined. This optimum range is related to the emissivity ofthe radiation shields, and to the weight per unit area of the web layerused to separate the shields.

lf the insulation is compressed excessively so as to install more thanthe optimum number of layers per unit thickness, then heat conductanceincreases sharply due to additional solid material being present totransfer the heat. The fibers are thereby crushed and matted to such` adegree that numerous fiber-to-ber contacts result in excess of thenumber needed for web strength and for radiation shield supportpurposes. On the other hand, if the composite insulation is relaxedexcessively by installing yfewer than the optimum number of layers perunit thickness, the heat transmission increases rapidly due to thedecrease in the number of radiation shields per unit thickness and dueto the increase in gaseous conductance. Gaseous conduction increasesbecause the voids become larger, thus permitting greater freedom ofmolecular motion.

FIG. 8 illustrates the very pronounced effect of varying the density ofthe web materials by applying different degrees of compression duringinstallation. Curve A correlates installed web density pf with theportion ksc of the heat transmission due solely to solid conductionthrough the fibers, and is represented by the following empiricalequation:

ksc=11 10*5(pf)2" (l) which may be written in the alternative form keu.416 p" 1.1 1o5 (2) The steep slope of the curve indicates thesensitivity of the web to compression, and illustrates the detrimentaleffect of crowding too many layers in a given space or of requiring thecomposite insulation to support a large sustained load in service asdiscussed later in detail. Curves B1, B2 and B3 show the change inradiant heat transmission which normally accompanies a change in webdensity. As the insulation is compressed, the separating weblayersbecome thinner fand more shields can be installed in a unit thickness ofcomposite insulation with the result that heat transfer by radiation isdecreased. The B` curves are determined by the general formula:

where kr is the component of the total heat transmission due toradiation, e is the effective emissivity of the foil surface in theassembled condition, y is weight per unit area of the web layer ingrams/ sq. ft., and pf is the installed web density in lbs./ cu. ft.Thus, each of the B curves is typical for a given foil emissivity e anda given weight per unit area of the web layer. For example, curve B2 issuitable for a web layer of 4.7 grams per sq. ft. used with a yfoilemissivity of .021 for which the product efy is .02l 4.7=0.l0. If forexample an installed web density pf of 3.0 lbs/cu. ft. is employed,curve B2 indicates that the contribution of radiation to the total heattransmission is 0.0l4 1103 B.t.u./(hr.) (ft.) F.). The number of layersof foil and web materials which must be installed per inch of compositeinsulation thickness in the abt/e examples is pf/ or (3.0/4.7)(453.6/12) :24 layers/ mc It should be emphasized that fy is defined asthe weight per unit area of the total web layer used to separateadjacent foils. Thus web material available in sheets weighing 2 gms/sq.ft. will have a value 'y of 2.0 if used singly between foils, and avalue of 4.0 if used in a double thickness between foils.

Curves C1, C2 and C3 are the sums of heat transmission by solidconduction and by radiation, e.g., C1 is the sum of A and B1. Assumingthat heat transmission by gaseous conduction is negligible, the C curvestherefore represent total :heat conduction K,L for the insulation. It isentirely proper to assume that gaseous conduction will be negligible forthe high quality insulations of this invention wherein heat transfer byall modes isminimized. In order to justify the installation of a largenumber of radiation shields, it is first necessary to essentiallyeliminate gaseous conduction by employing :a suitable Vacuum and smallfiber diameter web material so that, without shields, radiation becomesIa major contributor to total heat transmission.

The C curves exhibit definite minimums with extensions which approachthe radiation curve B on the left and the solid conductance curve A onthe right. It is apparent that unless the compression sensitivity of theweb materials is recognized and properly used in accordance with thepresent invention, the composite insu-lating quality may be only afraction of that which may be obtained by operating in the minimum areaof the C curves. The C curves also illustrate the highly detrimentalelfectof using the composite insulation to withstand sustained physicalloads in service. The occasional practice of requiring the insulation tosupport the weight of the inner vessel is precluded for the presentinsulation since the latter would be compressed excessively beneath thevessel and would be too loose above the vessel. The common practice ofallowing the insulation to support or brace the walls of the vacuumspace against sustained atmospheric pressure force is also to beavoided, that is, the composite multi-layered insulation of the presentinvention is external load-free.

Optimum web density may be obtained by differentiating the sum ofEquations l and 2 with respect to density pf, and the following equationis obtained:

Additional algebraic manipulation leads to an expression for Ka minimumas follows:

Solution of Equations 3 and 4 to eliminate e7 provides the expressionfor curve D of FIG. 8 as follows:

Returning to the illustrativeexample using insulation materialscorresponding to curves B2 and C2, it will be seen that the selecteddensity pf of 3.0 lb.`/ft.3 is by no means optimum for the installedcomposite insulation. A density of 3.0 results in an overall heattransfer coeicient of about 0.17)(10-3 B.t.u./(hr.)(ft.)( F.), whereasthe materials are capable of providing a coefficient of 0.05 10"3B.t.u./ (hr.) (ft.)( F.) ifr installed with an optimum density of 1.2lb./ft.3.

FIG. 9 is a graph of Equations 4 and 5k which permits the designer toselect materialsy and to install them properly so that he may obtain arequired Ka value in the most economical manner. Assuming any desiredoverallheat transfer coetiicient Ka, the necessary emissivity-to-webweight relationship can be determined from curve A of FIG. 9. Also, theproper installed density for the yweb material can be determined fromcurve B. Once the web weight is selected, the number of layers per unitthickness (N) may be calculated using the optimum installed density:

N=(pf/fy)(453.6/12) (7) For preferred practice of the invention, apractical limitation is imposed by the physical characteristics of thematerials. From FIG. 8, one might infer that ultimately, bestperformance Would be obtained at extremely low densities below thoseincluded on the curves, provided that materials can be found of suitablyllow emissivity and web weight. However, the insulation must becompressed suliiciently to prevent sagging and excessive wrinkling, and'to maintain contact between the webs and the shields. Sagging andwrinkling produces large gross voids within the insulation which take upspace in the insulation compartment and contribute little to theinsulating effect. Since Ka ratings for insulations are based on theireffectiveness per unit thickness, it will be apparent that sagging orexcessive wrinkling will seriously reduce the Ka value. Contact isdesirable to produce sufficient friction between the low conductivelayers and reflective shields so that the composite insulation may behandled easily and without damage during assembly of equipment such asinsulated containers. In order to avoid difficulty, it has been foundthat the installed density of the web material should be not less than0.5 lb. per cu. ft. The heavy vertical dashed line on FIG. 8 at pf=0.5defines this preferred boundary.

The horizontal dashed line on FIG. 8 representing Ka=0.8 l03 approachesthe best practical insulation of the prior art, i.e., powder-in-vacuuminsulationand represents the upper limit of applicability of thisinvention. Powders are normally available in finer particle size thanwebs, and for systems permitting Ka values above about 0.8 10-3, powdersare preferable in order to utilize their lower gas pressure sensitivity.

The superiority of the web-type alternate layer insulations of thisinvention is shown clearly by comparison with points E, F, and G on FIG.8. Point E represents typical performance of fine perlitepowder-in-vacuum, while point F represents performance of a sub-micronparticle size silica aerogel in vacuum. Point G is the performance of aclean glass cloth woven of 5-6 micron diameter libers and having aweight of about 2.6 grams per sq. ft. T his cloth was tested inalternate layers with aluminum foil in the same manner as the web-typematerials. In woven materials, the fibers are not randomly oriented inthe plane of the sheet, but instead bundles of fibers in closelongitudinal contact pass alternately from side to side through thesheet. This results in a very'high density material which exhibits veryhigh heat conductance through the fibers. Due to the openness of theweave, cloth materials are also very gas pressure sens1t1ve.

It is to be understood lthat employment of reflective shields with theprior art powder materials under a vacuum would not provide a workableinsulating system. This is primarily due to the iiuid and settlingcharacterlstics of powders, which are magnified by the movement andvibration associated with portable systems. Also, foam-type materialsand coarse, non-oriented or bonded fibers exhibit such high solidconductance that even used with radiation shields, would produce thermalconductivities well above the range of this invention. Similarly,non-vacuum insulating systems of all types are characterized byextremely high thermal conductivities due to the overwhelmingcontribution of gaseous conduction.

As may be concluded from the previous discussion an important advantageof the insulation of this invention 1s the very low coefiicients of heattransmission which may be obtained. For example, using an insulationconsisting of alternate layers of aluminum foil having an effectiveemissivity of 0.05 8 and a 4.7 gin/sq. ft. web of oriented, unbondedtype B compressible glass fiber, a thermal conductivity coefficient ofO.ll8 10lP3 B.t.n./ (hr.) (ft.) F.) has been obtained at a near-optimumweb density of 1.6 lbs./ cu. ft. If the illustrative double-walledcontainer described above were insulated "with this material, aninsulation thickness of only 1.31l inches would be required for anevaporation rate of 1% per day or contained liquid oxygen. In order tofurther demonstrate the effectiveness of this insulation, Table Icompares its thermal conductivity with that of the prior artinsulations. f

Table I Absolute Thermal Pressure Conductiv- Iype of Insulation inVacuum ity B.t.u./

Space (hr.) (ft.) Microns F.) Mercury Powder-in-vacuum insulatingsystems in accordance with U.S. Patent 2,396,459 O. 1 D. 2 10-4 Highvacuum-polished metal surface sys- 1 tern with radiation shields inaccordance with U.S. Patent 2,643,022 0.01 1. 9)(10-4 Insulationillustrative of this invention: B fiber web 4.7 gms/sq. ft. alternatingwith 94.2% reflective aluminum foil compressed to a fiber density of 1.6lbs/cu. ft 0. 1 1. 18Xl04 It is thus seen that the quality of thepresent insulation is over eight times that of the poWder-in-vacuumtype. Compared with the high vacuum-polished surface type, thisinvention reduces the conductivity more than 60% and simultaneouslypermits use of a practical vacuum. By using lfoils of higherreflectivity and webs of lighter weight, still lower coefficients ofthermal conductivity are obtainable.

Another advantage of the Web-type alternate layer -insulation is its lowweight per unit heat ow resistance. This is an important characteristicfor two reasons: first, it achieves minimum tare weight in portablecontainers, and thus facilitates handling and reduces transportationcosts; second, by minimizing the insulation weight one also reduces theamount of expensive refrigeration needed -for cooling the inner vesselto operating temperature and for establishing a stable temperaturegradient through the insulation thickness. An insulations weight perunit heat flow resistance is measured by the factor (k) (p), where k isthe coefficient of heat transmission (reciprocal of heat resistivity)and p is the total density of the material including shields. Forexample the alternate-layer insulation described `above with acoefficient of 0.118X10-3 B.t.u./(hr.)(ft.)(-F.) was found to have atotal ydensity p of 2.5 lbs/cu. ft. Thus its (k) (p) factor is 0.29Xl03,but the present invention contemplates factors as high as 1x10-3. Bycontrast the (k) (p) factor for perlite-in-vacuum is about 9.6 1()-3 andthat for sub-micron size silica aerogel in vacuum is about 6.0X 3. Analternate layer insulation using aluminum foil with a woven glass clothwas found to have a (k) (p) factor of 2.75 10-3, still about l0 'foldgreater than the illustrative web and foil insulation of this invention.

As k values are reduced, low total density becomes increasinglyimportant in order to obtain a reasonable cool-down time for theinsulation when placed in service. Reference to point G located on FIG.8 shows that while woven glass cloth with foils achieves a significantimprovement in k-factor over the prior art, it unfortunately has anextremely high density (22.7 lbs./ft.3 density of fiber alone). If awarm container insulated with a 3-inch thickness of such insulation werefilled with liquid nitrogen, about 1200 hours time is required toapproach within 10% of a steady temperature gradient. On the other handif 3 inches of a low density web and foil insulation is used, only 600hours time is required to reach an equivalent gradient in spite of thefact that the k value is approximately 1/3 that of the glass cloth andfoil insulation. A thickness of 9 inches of the glass cloth and foilinsulation would be required to achieve a heat flow resistance equal to3 inches of web and foil, but such a bulky insulation would take over10,000 hours to apprach a stable gradient.

Another 'of the many important advantages in the thermal insulation ofthe present invention is that the flexibility of the layers of aluminumfoil and fiber glass web allows the insulation thickness as a whole tobe pliably bent so as to conform to irregularities and changes in thesurface conditionsof the container to be insulated. The compositematerial of the invention is adapted to be applied to contouredsurfaces, and is particularly well suited for insulating either flat orcylindrical surfaces.

Obviously the multiple foil insulation of the invention may be mountedin the insulation space in any one of a variety of ways. For example, inFIG. 5, the insulation 12 may be mounted concentrically with respect tothe inner container 10a, or it may be, as in FIG. 4, spirally wrappedaround the inner vessel with one end of the insulation wrapping incontact with the inner vessel 10a,

0 and the other end nearest the outer casing 10b or in actual contacttherewith, the latter form of mounting being preferred and illustratedherein. Referring to FIG. 4, the metal foil may be loosely spirallywrapped around the inner Vessel 10a, the tightness and number of turnsbeing selected preferably to obtain optimum performance as discussedabove.

It will be recognized that because of the difficulty involved inconformably applying the composite insulation material 12 of theinvention to surfaces other than iiat or cylindrical surfaces withoutsacrificing insulating qualities, for maximum benefit it may beadvantageous in some instances to employ a supplementary low heatconductive material in combination with the insulation 12.

In the modification shown in FIG. 6 the composite insulation material 12of the invention may be employed in the cylindrical portion 11a of theinsulation space 11, and the end portions 11b of the insulation space,including the flat bottom portion and the upper spherical portion,provided with a supplemental low heat conductive material 16. Thesupplemental low heat conductive materials which may be used in theterminal sections 11b may comprise a finely divided powder of the typedisclosed in U.S. Patent No. 2,396,459, or a thermal insulation such asdisclosed in the co-pending application to L. C. Matsch et al., SerialNo. 580,897, filed April 26, 1956, now Patent No. 2,967,152, or anyother suitably low conductive material.

Coupled with the composite insulation 12, the supplemental insulation 16provides the means for producing low thermal heat transfers incontainers of a wide variety of shapes. The cooperative relationshipbetween the supplemental insulation 16 and the composite insulation 12meets the requirements of the most critical present day insulationstandards, and has extended the usefulness and capabilities of thepresent invention.

A very significant advantage of the present invention arises from theelastic properties of the insulation, particularly when a fibrousinsulation is employed in the annular insulating space of a doublewalled container. The ability of the insulation to give and resistmovement of the inner container, and to restore or expand itself whenthe forces exerted upon it are relaxed, enables -it to operate along thelines of a shock mount. Obvious advantages to using the insulation as anelastic support are that the inner vessel is maintained in substantiallycentered position, and the need for lateral braces or other centeringdevices is obviated, thus further reducing the heat vleak into thecontainer. It is to be understood, however, that the present inventiondoes not provide vertical support for the inner vessel, and thatspecific means for vertical support must be provided.

In the preferred modification shown in FIG. 7, means are provided forfacilitating the evacuation of the insulating space between walls l10nand 110k after the composite insulation of the invention has beeninstalled therein. For this purpose, the aluminum foil 114 is providedwith passages or perforations 115 preferably arranged in helical orspiral rows, the perforations in any one layer of foil being out ofregistry with the perforations in the adjacent foil layers. Thisarrangement affords means for providing a suitable number ofperforations in the foil 114 without noticeably reducing the shieldingproperties thereof. Suitable perforations are 1/16 in. diameter holespricked or punched on 11/2 in. centers.

The function of the perforations is to permit gas in the insulationspace to move radially through the insulation layers, rather than onlyparallel to the foil layers. This permits the gas molecules to migratemore freely towards the evacuation connection or toward a gas trap pingmeans such as an adsorbent or getter. Also, the perforations permit themigration of cO-ndensable gases to the cold outer wall 110a of the innervessel, where they form deposits having negligible vapor pressure.Perforations are especially beneficial in systems where the insulationdoes not fill the entire space but is omitted from one or more free flowchannels running :from the evacuation connection to remote points withinthe insulation space. For example in FIG. 7, space 116 between theouter-most foil layer 113:1 and the outer wall 110i; of the vacuum spacecomprises an open channel leading to an evacuation connection (notshown). Gas molecules trapped near cold wall ln are free to migratethrough the holes 115 in foils 1114 and through webs 113 until theyreach low flow resistance channel 116.

From the above description it will, therefore, be seen that the presentinvention provides in a solid-in-vacuum type insulation, a compressiblelow heat conductive material having incorporated therein multipleradiation shields for impeding radiative heat transmission through theinsulation, while minimizing the lflow of heat by conductiontherethrough. The low conductive material uniformly supports andmaintains the radiation shields in spaced relation. A low conductivematerial which is admirably suited for use in the practice of theinvention is one having a fibrous structure oriented in a directionperpendicular to the direction of heat flow. Possessed of a relativelysmall percentage of solid material per unit volume, the low conductiveinsulating material provides a very small, solid conduction heat pathbetween radiation foils, and is remarkably efiicient in minimizing thetransmission of heat leak by conduction.

insulating systems of the invention, using a fine diameter, -lowconductive, compressible fiber-type insulating material, have been foundto be superior to any lmown insulating system. Employing coarser,lou/conductive insulating fibers, the present insulation achieves lowthermal conductivities, which are comparable or superior to thoseobtained with either high quality straight vacuums or the bestpoWder-in-vacuum systems known, yet is considerably less expensive thaneither of these forms of insulation, and does not require as lowabsolute pressures as straight vacuum-polished metal insulating systems.

It will be understood that variations and modifications may be effectedwithout departing from the novel concepts of the present invention. Forexample, independent support means such as high strength plasticspacers, may be provided in the insulation space to support the wallsdefining such space from the load of atmospheric pressure.

This is a continuation-impart application of my application Serial No.597,947, filed July 16, 1956.

What is claimed is:

l. In an apparatus provided with a vacuum insulating space, a compositemultielayered, external load-free insulation in said space comprisinglow conductive fibrous sheet material Vlayers composed of fibers forreducing heat transfer by gaseous conduction and thin, iieXible radiantheat refiecting shields, said radiant heat reiiecting shields beingsupportably carried in superposed relation by said fibrous sheet layersto provide a large number of radiant heat reliecting shields in alimited space for reducing the transmission of radiant heat across saidspace without perceptively increasing the heat transmission by solidconduction thereacross, each radiant heat reflecting shield beingdisposed in contiguous relation on opposite sides with a layer of thefibrous sheet material, the fibers of said fibrous sheet material beingoriented substantially perpendicular to the direction of heat inleakacross the insulating space, said fibrous sheet material being anelastically compressible web composed of fibers having diameters betweenabout 0.2 and 5 microns, said radiant heat refiecting shields having athickness less than abou-t 0.2 mm. and fbeing perforated to provide fiowpaths through the shields, and said multi-layered composite insulation`being generally spirally wound in the insulation space to provide morethan 5 radiant heat refiecting shields per inch of said compositeinsulation.

2. In an apparatus provided with a vacuum insulating space, a compositemulti-layered, external load-free insulation in said space comprisinglow conductive fibrous sheet material layers composed of fibers forreducing heat transfer by gaseous conduction and thin, fieXible radiantheat retlecting shields, said radiant heat reiiecting shields beingsupportably carried in superposed relation by said fibrous sheet layersto provide a large number of radiant heat reflecting shields in alimited space for reducing the transmission of radiant heat across saidspace without perceptively increasing the heat transmission by solidconduction thereacross, each radiant heat reflecting shield beingdisposed in contiguous relation on opposite sides with a layer of thefibrous sheet material, the fibers of said fibrous sheet material beingoriented substantially perpendicular to the direction of heat inleakacross the insulating space, said fibrous sheet material being anelastically compressible web composed of fibers having diameters betweenabout 0.2 and 5 microns, said radiant heat reflecting shields having athickness less than about 0.2 mm. and being perforated to provide flowpaths through the shields, and said multilayered composite insulationbeing disposed in the insulation space to provide more than 5 radiantheat reflecting shields per inch of said composite insulation.

References Cited in the tile of this patent UNl'TED STATES PATENTS903,878 Mock Nov. 17, 1908 1,626,655 Woodson May 3, 1927 2,104,548Schweller Jan. 4, 1938 2,150,182 Munters Mar. 14, 1939 2,345,204 LodwigMar. 28, 1944 2,676,773 Sanz et al. Apr. 27, 1954 2,776,776 Strong etal. Jan. 8, 1957 FOREIGN PATENTS 143,219 Great Britain Dec. 9, 1920683,855 Great Britain Dec. 3, 1952 715,174 Great Britain Sept. 8, 1954

